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If you’re experiencing unusual changes in your menstrual cycle, you’re not alone. An estimated 14% to 25% of women report irregular menstrual cycles, also called abnormal uterine bleeding.
There are many potential causes of abnormal uterine bleeding, some of which you may already be familiar with. But did you know growing evidence supports the link between abnormal uterine bleeding and hypothyroidism?
You might wonder how your thyroid hormone levels relate to your monthly cycle. Well, let’s dive in and explore the connection.
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Every month, a woman’s body goes through hormonal changes to prepare the body for a possible pregnancy. If pregnancy does not occur, the levels of your reproductive hormones fall. Because of this, your uterine lining breaks down, causing menstrual bleeding.
A “regular” menstrual cycle ranges from 21 to 35 days, with menstrual bleeding lasting 4 to 7 days. But everyone’s menstrual cycles are different; what is normal for you might not be the same for someone else.
Abnormal uterine bleeding (abbreviated as AUB) describes irregularities in a woman’s menstrual cycle. There are many types of AUB, including the following:
- Amenorrhea: not having your period for three months or longer
- Dysmenorrhea: painful periods
- Menorrhagia: heavy or prolonged bleeding
- Metrorrhagia: bleeding between periods
- Oligomenorrhea: infrequent menstrual periods occurring more than 35 days apart
- Polymenorrhea: frequent menstrual cycles occurring less than 21 days apart
Your menstrual cycle requires a delicate balance of your hormones, including estrogen and progesterone. But, an imbalance in other hormones not directly related to your menstrual cycle can influence reproductive hormone levels and cause AUB.
Besides hormonal imbalances, other common causes of AUB include:
- Structural changes in your reproductive organs: Abnormalities within your reproductive system, such as uterine fibroids or polyps, may interfere with the function of your uterus.
- Medications: AUB is a side effect of certain medications such as hormonal contraception like birth control pills.
- Underlying medical conditions: Medical conditions, including polycystic ovarian syndrome (PCOS), uterine cancer, or hypothyroidism, may alter your hormone balance.
- Stress: Chronic stress can disrupt many hormonal balances in your body, and your reproductive hormones are no exception.
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The thyroid hormones thyroxine (T4) and triiodothyronine (T3) are essential for maintaining your overall health. One of the critical roles of these thyroid hormones is to regulate your metabolism by ensuring every cell receives the energy needed to function correctly. Having too much or too little thyroid hormone affects how your cells work.
Symptoms of hypothyroidism
When you don’t have enough thyroid hormone (hypothyroidism), your metabolism slows down. Low thyroid hormone levels result in symptoms of hypothyroidism, including:
- Anxiety or depression
- Bloating
- Brain fog
- Constipation
- Fatigue
- Weight gain
A lesser-known symptom of hypothyroidism is AUB.
A 2018 study screened 100 perimenopausal women with a provisional diagnosis of AUB for thyroid dysfunction. According to their results, 32% of the women had a form of thyroid dysfunction, with the majority having either subclinical (mild form) or overt hypothyroidism. A very small portion had hyperthyroidism or high thyroid hormone levels.
A more recent 2021 study showed similar results to the 2018 study. Most women in this study with AUB and thyroid dysfunction were found to have overt hypothyroidism.
This begs the question: what is the link between hypothyroidism and abnormal uterine bleeding?
The above 2018 study found that polymenorrhea and menorrhagia were the most common types of AUB seen in those with subclinical or overt hypothyroidism. Experts suspect the link between hypothyroidism and AUB is related to an overlap in the complex feedback loops that regulate these hormones, amongst other potential causes.
Feedback loops
Separate feedback loops regulate your thyroid and reproductive hormone levels. But, both feedback loops begin in your brain, specifically the hypothalamus, with the release of thyrotropin-releasing hormone (TRH).
The release of TRH stimulates your pituitary gland to release thyroid-stimulating hormone (TSH) and prolactin.
TSH is part of the hypothalamus-pituitary-thyroid feedback loop regulating thyroid hormone levels. When released, TSH tells your thyroid gland to secrete thyroid hormone. Once there is enough thyroid hormone in your bloodstream, TSH production decreases.
Those with hypothyroidism have high levels of TSH because your pituitary gland continuously secretes TSH to raise thyroid hormone levels.
On the other hand, prolactin, nicknamed the “milk hormone,” is part of another feedback loop. Prolactin’s primary role is to stimulate breast milk production after childbirth. High prolactin levels, a term called hyperprolactinemia, can cause hormonal imbalances in your reproductive hormones, such as follicle-stimulating hormone (FSH) or luteinizing hormone (LH). Because of this, hyperprolactinemia can result in AUB, with amenorrhea being the most common.
While prolactin and TSH belong to different feedback loops, one influences the other.
Clotting factors
Hypothyroidism can also interfere with your blood clotting factors, and your blood can take longer to clot. As a result, menorrhagia can occur. Those taking blood thinners such as warfarin may also be at a higher risk for AUB.
If your AUB is due to hypothyroidism, there is good news. By correcting your thyroid hormone imbalance, AUB should resolve.
To do this, most will need prescription thyroid hormone replacement medication. These medications mimic the structure and function of your natural thyroid hormone. Thus, they help to bring thyroid hormone levels within a normal range. By increasing your thyroid hormone, your hypothalamus will decrease TRH release, thus decreasing TSH and prolactin release.
It can take up to 8 weeks for thyroid hormone replacement medications to become fully effective. And you will likely need dose adjustments, especially in the beginning. During this time, you may still experience AUB. Once you are on a stable dose, AUB should subside.
The American College of Obstetrics and Gynecology recommends checking a TSH level in AUB patients. If you have AUB and haven’t checked your TSH, speak with your healthcare provider.
There are two ways to check your thyroid function: going to the lab or doctor’s office for a blood draw or using a home testing kit. Paloma Health’s at-home testing kit allows you to measure three thyroid biomarkers -- TSH, free T4, and free T3 -- to assess your thyroid function. Based on these three levels, your healthcare provider can determine if a thyroid disorder is a contributing cause of your AUB.
If your thyroid biomarkers are abnormal, Paloma has an expert team of thyroid specialists who can help you develop a comprehensive, personalized treatment plan.
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